1. Field of the Invention
The present invention relates to a circuit board fabricated by forming a circuit pattern having a multilayered structure on a ceramic substrate.
2. Description of the Related Art
A circuit board having a structure in which a circuit pattern is formed on a ceramic substrate is used by mounting, e.g., semiconductor chips on it. Recently, the integration degree, and power consumption of semiconductor chips, such as LSI chips and VLSI chips, are increasing. Therefore, a circuit board for mounting semiconductor chips, such as LSI chips, is required to have a higher adhesion between the ceramic substrate and the circuit pattern, a lower resistance of the circuit pattern, and better heat removal properties of the board than before. Also, in order to mount semiconductor chips, pin bonding regions are formed on the circuit board, and a large number of leads (e.g., pins) are bonded by using a brazing material.
A conventionally known example of a circuit board of this type is disclosed in Published Unexamined Japanese Patent Application No. 1-223737. This circuit board is fabricated by forming, on the surface of a ceramic substrate consisting of an aluminum nitride (AlN) sintered material, a circuit pattern having a multilayered structure in which a first layer consisting of at least one element selected from the group consisting of Ti, Cr, Mo, and W, a second layer consisting of Ni or Cu, and a third layer consisting of Au are stacked in sequence.
If, however, the circuit board having a circuit pattern constituted by a multilayered structure of, e.g., Ti/Ni/Au, is exposed to heat during bonding of pins or packaging of semiconductor chips, the individual layer elements of the circuit pattern diffuse each other to increase the resistance significantly.
Published Unexamined Japanese Patent Application No. 64-84648 discloses a circuit board fabricated by forming a circuit pattern having a two-layered structure of Ti/Cu on a ceramic substrate consisting of an AlN sintered material. In this circuit board, however, the adhesion strength between the ceramic substrate and the circuit pattern is insufficient. In addition, when the circuit board is accepted a thermal influence, the individual elements of the circuit pattern diffuse each other to increase the resistance.